Darlington amplifiers are noted for having a wide band gain response, a compact size, and a low cost package implementation. Darlington amplifiers typically have superior gain-bandwidth performance compared to common-emitter feedback amplifiers and/or are popular RF gain block products.
While Darlington amplifiers have advantages over the common-emitter transistor in conventional feedback amplifiers, the broadband linearity performance of a Darlington still suffers due to the open loop insertion phase departure from 180 degrees as frequency increases. As a result, the IP3 (or third order intercept point) of the Darlington feedback amplifier, a measure of amplifier linearity, uniformly degrades with increasing frequency.
Referring to FIG. 1, a circuit 10 illustrating a conventional Darlington feedback amplifier topology is shown. The circuit 10 comprises a Darlington transistor pair 12 (implemented with a transistor Q1 and a transistor Q2), a series feedback resistor RE2, a parallel feedback resistor RFB, and bias resistors RE1 and RBB.
The Darlington pair (or cell) 12 is known to have advantages over the common-emitter transistor in feedback amplifiers applications. For instance, the Darlington amplifier 10 can provide higher current gain and cut-off frequency, and can be designed to have a higher input impedance, which is preferred in feedback implementations.
It would be desirable to implement a circuit and/or method to compensate for the fundamental phase degradation of the Darlington cell. It would also be desirable to use such a circuit and/or method to improve broadband IP3 performance by optimizing the second harmonic phase response of the Darlington cell.